Aircraft landing aids, particularly microwave landing systems, must meet very stringent accuracy requirements. To be able to satisfy these requirements, the antennas used must be very well calibrated. This applies to both azimuth antennas (AZ antennas) and elevation antennas (EL antennas). U.S. Pat. No. 4,502,361 discloses a method of calibrating a phased-array AZ antenna with 4-bit phase resolution wherein probes are inserted into each individual waveguide radiator. It has been found, however, that in phased-array antennas with 6-bit resolution, the reproducibility of the measurements with the aid of probes does not yield satisfactory results. Such an antenna could be better calibrated if its aperture amplitude and phase illumination were known. To derive the aperture illumination of a phased-array antenna, use is made of integral monitor waveguides. Signal components from each radiating element are coupled through coupling holes into an integral monitor waveguide either shortly before or immediately after transmission. The output of the integral monitor waveguide corresponds, to a first degree of approximation, to the far-field pattern of the antenna. The far-field pattern and the antenna aperture illumination are related by a Fourier transform. Therefore, the complex aperture illumination of the antenna can be determined from the output of the integral monitor waveguide. A conventional method of doing this is the quadrature method (I/Q converter). In this method, the signal from a local oscillator is mixed with the output signal from the integral monitor waveguide twice, once at an angle of 0.degree. and a second time with a 90.degree. phase shift. The mixing with a 0.degree. phase shift provides the real part of the output signal of the integral monitor waveguide, and the mixing with a 90.degree. phase shift provides the imaginary part. A subsequent Fourier transformation of the real and imaginary parts of the output signal yields the aperture illumination of the antenna. A disadvantage of this method is the use of two mixers.
It is the object of the invention to provide a method of and an apparatus for calibrating phased-array antennas in a reproducible manner and with an accuracy required to meet safety standards.